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Changes for version 1.40

Updated version of examples/data_gathering/* scripts, as currently used on http://rrd.me.uk

Changed RRDs::error calls to RRDs::error().

Changed debug warn() calls to carp().

Explicitly import fewer symbols from other modules.

Added rename_source() method and associated POD and unit test.

Changed the graph() method to recognise use of command (GPRINT,PRINT,COMMENT,VRULE,.. etc) over options and flags, so that they will be added to the graph definition after the data sources have been defined.

Added new unit tests for new functionality. t/35average_hrule.t tests and demonstrates how the changes to the graph() method can be used.

Fixed a bug where an uninitialised variable could cause unexpected behaviour if you tried to add a new data source using add_source() method. This only manifested itself with an versions of RRDs earlier than 1.2013, and where the rrdtool binary is not installed or the path to the rrdtool binary is unknown.